Photovoltaic devices are used to convert light into electrical energy. Photovoltaic devices are characterized by their efficiency with which they can convert incidental light to useful electrical energy.
Traditionally, photovoltaic devices have been made of various inorganic semiconductors, e.g., crystalline, polycrystalline, and amorphous silicon, gallium arsenide, cadmium telluride, and others. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved high efficiencies. However, it is difficult and expensive to produce efficient crystalline-based devices, especially of large surface area, due to the inherent difficulties in producing large crystals without significant efficiency-degrading defects. Further, high efficiency amorphous silicon devices have problems with stability.
Recently, organic photovoltaic (OPV) devices are being researched and developed to achieve acceptable photovoltaic conversion efficiencies with economical production costs. However, since the energy conversion efficiencies of OPV devices are still low in comparison to their inorganic counterparts due to mainly very weak molecular orbital coupling and subsequent low carrier mobility of organic semiconductor, the industrial production of organic solar cells is not yet economical. One promising approach to circumvent the problem of low carrier mobility and small exciton diffusion bottleneck and thus to increase the overall energy conversion efficiencies of organic solar cells is the utilization of hetrojunction OPV or dispersed heterojunction OPV with a donor-acceptor system, consisting of n- and p-type conducting materials in a double layers separately (hetrojunction OPV) or a single layer prepared by blending the two n- and p-type materials (dispersed heterojunction OPV). The success and efficiency of dispersed hetrojunction OPV is largely depend on the domain size of interpenetrating donor-acceptor network whose size is ideally in the range of 10 nm. Since the degree of interpenetrating phase separation and the domain size depend on the choice of the solvent, speed of evaporation, solubility, miscibility of the donor and acceptor, etc., controlling the morphology of the organic compound in dispersed heterojunction devices is critical.
One way of controlling the morphology of an organic compound is to produce a molecular heterojunction by covalently linking electron-donor and acceptor molecules. Due to their unique optical and electrical properties, molecular heterojunction materials, i.e., donor-acceptor linked molecules, are likely to have important applications in OPV devices and, thus, have triggered intense scientific research.
Among the electron-donor and acceptor materials that are capable of producing a molecular heterojunction, oligothiophene and perylene tetracarboximide functional units have been extensively studied since the former preserves its typical charge-transport and self-assembling properties, while the latter provides high molar absorptivity in the visible region as well as electron-accepting properties. For example, Cremer et al., “Perylene-Oligothiophene-Perylene Triads for Photovoltaic Applications,” Eur. J. Org. Chem., 3715-3723 (2005) discloses acceptor-donor-acceptor triad systems consisting of head-to-tail-coupled oligo(3-hexylthiophenes) integrated between two terminal perylenemonoimides which can be used for organic solar cells. In addition, Chen et al., “Oligothiophene-Functionalized Perylene Bisimide System: Synthesis, Characterization, and Electrochemical Polymerization Properties,” Chem. Mater., 17:2208-2215 (2005) describes perylene bisimides derivatives functionalized with two oligothiophene substituents. Further, Xiaowei et al., “A High-Mobility Electron-Transport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells,” J. Am. Chem. Soc. 129:7246-7247 (2007) discloses a copolymer of perylene diimide and dithienothiophene building blocks exhibiting broad absorption ranging from the visible to the near infrared regions. Huang et al., “Size Effects of Oligothiphene on the Dynamics of Electron Transfer in π-Conjugated Oligothiophene-Perylene Bisimide Dyads,” J. Phys. Chem. C 112:2689-2696 (2008) discloses the preparation of a series of π-conjugated perylene bisimide dyads having two oligothiophene moieties, while PCT International Publication No. WO 08012584A describes several perylene tetracarboximides as hole-transporting materials for light emitting devices.
However, none of the above compounds disclosed in the art exhibits sufficiently high efficiency, charge carrier mobility, or stability when utilized in OPV devices. It would thus be desirable to develop perylene tetracarboximide derivatives that have potentials for ideal electron donor for fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), high photovoltaic conversion efficiencies and are stable for OPV device applications.